The present invention relates to a hollow roller suitable for a power transmission of a planet roller type or a roller bearing.
Conventionally, when a hollow roller adapted for a power transmission of a planet type roller or a roller bearing is made of bearing steel, the surface of the steel is hardened by a carburizing process to enhance wear resistance of an outer surface when coming in contact with an opposing surface.
Conventionally, it has been considered that a soft portion should be retained at the inside of the thickness hollow roller to relax any shock.
If the hollow roller is thick enough, such a soft portion can remain. In the case of a hollow roller suitable for a compact roller bearing or a power transmission of a planet roller type which is rather thin, it is difficult to retain the soft portion at the inside of the thickness because it is difficult to precisely control the depth of carburization, in particular, due to the development of carburizing from both the outer surface and the inner side of the hollow roller. If the hollow roller is hardened from the inside and the soft portion is not retained there, tentative deformation due to shock is followed by concentration of tensile stress at the inner surface of the hollow roller, so that the portion may readily crack.
Therefore, conventionally, a rather thick hollow roller is carburized and, then, a hardened portion at the inside surface is ground to expose a soft portion at the inner surface of the hollow roller which is thin enough.
However, the inner side must be ground after the carburizing process, whereby the manufacturing steps become much to troublesome and costly.